Recently, demands for reducing the weight of the radial tire, for prolonging the life of the tire product by reduction of rolling resistance and enhancement of retread life, and for flattening the tire shape are required even in the heavy duty radial tires in compliance with the increase of social needs such as resource-saving, energy-saving and the like. If it is intended to develop tires along such demands, when metallic cords are used as a material for carcass ply, it is an important point to solve problems on fracture resistance, corrosion fatigue resistance and fretting resistance at an end of the carcass ply.
Therefore, from a viewpoint of input to the carcass ply cord, there have been attempted the formation of steel cord with two-layer twisting construction of 3+9 (Japanese Patent laid open No. 59-124,404) and the formation of compact cord structure with a closest packing construction of 1.times.12 (Japanese Patent Application No. 60-35,215) and the like as a method of reducing contact pressure between cord filaments. Further there is made examination on the twisting properties of these cords in order to improve the corrosion fatigue resistance and the fretting resistance (Japanese Patent laid open No. 59-124,404).
On the other hand, a large size radial tire for use on bad roads wherein cords having a single layer
construction of 1.times.4 or 1.times.5 obtained by twisting preliminarily formed filaments or a two-layer construction using 1.about.2 filaments as a core are used as a cord for an outermost belt layer is disclosed in Japanese Patent laid open No. 60-116,504.
The inventors have made various studies in order to develop heavy duty radial tires capable of largely improving the durable life and found that in case of the conventional 3+9 two-layer construction or 1.times.12 compact cord structure, the stress concentration to cord end is conspicuous in both ends of the carcass ply because rubber does not penetrate into the cord and hence fracture resistance is not sufficient at both ends of the carcass ply. Further, it has been found that it is difficult to penetrate rubber embedding the cord into the inside of the cord and a space inside the cord, i.e. a space surrounded by the filaments and opening in the axial direction of the cord is hardly clogged with the penetrating rubber so that it is impossible to move water penetrating from cut failure produced in the tread through the space formed in the inside of the cord over a far distance and the improvement of corrosion fatigue resistance is insufficient.
Moreover, as to the state of generating fretting (rubbing-out phenomenon due to the fretting of mutual filaments in the cord), the contact pressure between the filaments is high in the conventional 3+9 two-layer twisting construction or 1.times.12 compact cord structure, so that when the above cord construction is used as a cord for a carcass ply in radial tires for truck and bus (TBR), radial tires for light truck (LSR) or the like, fretting is caused owing to the severe input to induce the lowering of the strength and consequently the casing durability of the heavy duty radial tire is undesirably and largely degraded.
On the other hand, the technique disclosed in the above Japanese Patent laid open No. 60-116,504 is concerned with the improvement of the belt layer in the large size radial tire for use on bad road, which can not satisfy recently social demands such as a reduction of weight in the radial tire and the like. Further, when such a technique is applied to the carcass ply as it is, there are problems of bead durability and the casing strength, so that the application of this technique is impossible.
Furthermore, Japanese Patent laid open No. 57-51,502 discloses a pneumatic tire using as a reinforcement for carcass ply a steel cord of 7.times.4 twisting structure composed of steel filament wires constituting a greater part of the steel cord and containing 0.75.about.0.85% by weight of carbon and having a high tensile strength. However, this steel cord is a 7.times.4 strand construction, so that fretting between the strands is very large and the penetrability of rubber into the cord is poor. Consequently it does not match with the solution of the technical subject matter aiming at the invention.
It is, therefore, an object of the invention to provide an improving technique for a heavy duty radial tire which attempts a large reduction of tire weight, improves the fracture resistance, corrosion fatigue resistance and fretting resistance at the end of the carcass ply, which are problems in the above conventional technique, and can largely improve the performance of resistance to external damage in sidewall portion (resistance to cord breaking up) of the radial tire.